Allow admin user to change the base URL of FME Cloud account

Currently there is no facility to change the FME Cloud account URL after creating the account. Even though the documentation here suggests that the account URL can be altered by changing the Account Name, it doesn't work properly.

eg: if the current URL is https://<instance name>-<base url 1>.fmecloud.safe.com and if we need to change it to https://<instance name>-<base url 2>.fmecloud.safe.com

Therefore, it is worth to add the capability of changing the account URL at least for instances launched after the changes. It is awesome if the changes to URL can be applied to all the instances including existing instances.